1. Field of the Invention
The present invention relates to a stacked photovoltaic device having a plurality of photovoltaic cells each comprising a photoelectric conversion layer composed of a non-single crystalline semiconductor stacked therein and a method of manufacturing the same.
2. Description of the Background Art
In recent years, photovoltaic cells using thin film-based semiconductors such as amorphous silicon for photoelectric conversion layers have been developed. Amorphous silicon has features of abounding in raw materials, being low in manufacturing energy and manufacturing cost, making a wide variety of supporting substrates usable, making high voltages extractable, and easily increasing in area. On the contrary, the photovoltaic cells using amorphous silicon (hereinafter referred to as amorphous-based photovoltaic cells) are more greatly light-degraded, so that they have lower photoelectric conversion efficiencies, as compared with crystal-based photovoltaic cells.
On the other hand, photovoltaic cells using microcrystalline silicon for photoelectric conversion layers have been developed. The photovoltaic cells using microcrystalline silicon (hereinafter referred to as microcrystal-based photovoltaic cells) have photoelectric conversion efficiencies that are less reduced by light degradation and can absorb light in wider wavelength ranges, as compared with amorphous-based photovoltaic cells. In the microcrystal-based photovoltaic cells, therefore, photoelectric conversion efficiencies can be improved.
Stacked photovoltaic devices having amorphous-based photovoltaic cells and microcrystal-based photovoltaic cells stacked therein (tandem type photovoltaic devices or hybrid solar cells) have been developed (see JP 11-243218 A, for example). In the stacked photovoltaic devices, amorphous-based photovoltaic cells are arranged on the side of light incidence, microcrystal-based photovoltaic cells are arranged below the amorphous-based photovoltaic cells, and the amorphous-based photovoltaic cells and the microcrystal-based photovoltaic cells are connected in series. Such stacked photovoltaic devices can receive optical spectra in wide regions, so that photoelectric conversion efficiencies are improved. Consequently, the stacked photovoltaic devices are promising as high efficiency thin film solar cells for power use.
In the stacked photovoltaic devices, however, the amorphous-based photovoltaic cells are more greatly light-degraded by light irradiation, as compared with the microcrystal-based photovoltaic cells. Therefore, balances between the output characteristics of the amorphous-based photovoltaic cells and the output characteristics of the microcrystal-based photovoltaic cells are disrupted, so that the output characteristics of the whole stacked photovoltaic devices are deteriorated. As a result, the total power generations are low as observed in the long term.